1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus incorporating the optical scanning device, and, more particularly to an optical scanning device and an image forming apparatus employing the optical scanning device which performs an optical scan at a fast speed with a light source thereof remaining lit between scans, presents a high scanning efficiency, and is appropriate for use in a digital copying apparatus or a laser beam printer.
2. Description of the Related Art
Optical scanning devices have been widely used as optical writing systems in laser beam printers (LBP) or digital copying apparatuses. As these devices are in widespread use, high image-quality and high-speed requirements are mounting. For example, to achieve a high resolution of 600 dpi, a miniature spot of 60 xcexcm or so must be formed on a scanning surface. Since the diameter of a scanning beam becomes large, an optical deflector (polygon mirror) must be accordingly large. A large-scale polygon mirror needs to make a trade-off with the high-speed requirement.
A variety of proposals to achieve a high-speed operation have been made. For example, some of the proposed techniques include a multi-beam scanning method having an increased number of beams for performing parallel line scanning, and an OFS (Over-Filled Scanner) using an OFS scanning system having a small-diameter and multi-facet polygon mirror, as opposed to a conventional UFS (Under-Filled Scanner) using a UFS optical system.
The OFS is promising as a method for overcoming the following problems of a high-speed UFS method.
The UFS method, employing a polygon mirror, projects a light beam having a predetermined beam width to one deflection surface (reflective surface) of a polygon mirror, thereby forming a miniature spot having a spot diameter narrower than the width of the deflection surface in the main scan direction. As the deflection surface rotates in the UFS method, the arrival location of an incident light beam on the deflection surface moves. To keep the incident light beam free from shading within a predetermined scanning angle range, the width of the deflection surface in the main scan direction needs to be larger than a predetermined width. This leads to a large polygon mirror. If the number of deflection surfaces of the polygon mirror is increased, the polygon mirror becomes bulky, resulting in a large load in rotation. This presents difficulty in rotating the polygon mirror at a high speed. The UFS method typically implements a high-speed design with a multi-beam technique adopted. The multi-beam technique involves a complex construction.
The ratio of a theoretical scanning angle range, within which one deflection surface can scan a light beam, to a scanning angle range for scanning an effective scanning area is now referred to as a scanning efficiency. In the UFS method, the input light beam has a predetermined beam width, and the scanning angle range is determined on condition that the incident beam is free from shading on the deflection surface. The high-resolution UFS method using a large diameter beam typically offers a scanning efficiency of 70% or so. The remaining 30% is shared on portions prior to and subsequent to the image forming area, and is utilized to perform electrical processings such as stabilizing the output of the light source and detecting the write start timing of an image.
Subsequent to the end of the scanning of one line of the image forming area, the light source is extinguished. An undesired light beam not contributing to scanning can impinge on the end of an optical member or a structure supporting the optical member. The undesired light beam can then be reflected and scattered in a flare, which reaches the scanning area, thereby degrading the image. The extinction of the light beam is intended to prevent such image degradation from taking place.
At the scanning start end, the light source needs to be lit prior to the effective scanning area to detect the image write start position. The generation of similar flares is unavoidable. A light shield plate is installed or an optical member and a support member are properly configured for the flare not to reach the scanning surface even if such flare is generated.
In the OFS method, a light beam emitted from a light source is directed to a deflection surface of a polygon mirror with the beam width thereof being wider than the width of the deflection surface in the main scan direction (in other words, the light beam is incident on a plurality of deflection surfaces in a straddling fashion). One deflection surface thus rotates across the incident light beam. Since the width of the incident light beam is large enough, the incident light beam is not shaded by the scanning angle range of the polygon mirror. The width of the deflection surface may be set to be the same as the width of the incident light beam in the UFS system. Even if the number of deflection surfaces is increased, the diameter of the polygon mirror does not become as large as that in the UFS system. High-speed scanning is accomplished with the increased number of deflection surfaces.
Theoretically, the OFS system enables the scanning operation to function at a scanning efficiency of 100%. In practice, however, the scanning efficiency is restricted to 90% or so to assure time for the output of the light source to stabilize and time for detecting the image write timing prior to the start of the scanning.
With the number of deflection surfaces of the polygon mirror increased without enlarging the polygon mirror and the scanning efficiency increased, the OFS system enables a scanning device to scan at a high speed. With a multi-beam technique implemented, the OFS system permits an even higher speed scanning operation.
When a high-speed scanning feature is implemented, a scanning time for one line becomes short. A substantially high speed process must be performed for a short lighting preparation time prior to the scanning start. Process speed is determined by the performance of circuit control devices and the stability characteristic of a rising edge of the light source.
The process speed requirement for the OFS system is rigorous, because the OFS system has a scanning efficiency higher than that of the UFS system. There is no time available for the device to extinguish the light source subsequent to the end of one line of the effective scanning area, since the next line scanning starts soon. If the light source is extinguished, a rise time for the light source to reach a predetermined light intensity thereof is not assured.
Accuracies in the detecting of an image write start position prior to the line scanning start and the detecting of the output of the light source drop, thereby degrading a resulting image.
It is an object of the present invention to provide an optical scanning device and an image forming apparatus incorporating the optical device which includes a light shield member between an optical deflector and a scanning surface for blocking at least a portion of a light beam which is reflected and deflected by the optical deflector and scans outside an effective scanning area, thereby controlling flare light, wherein a light source remains lit during a period of time between the scanning of a plurality of lines on the effective scanning area.
The present invention in one aspect relates to an optical scanning device and includes a first optical system, including a light source, for directing a light beam emitted from the light source to an optical deflector, and a second optical system for focusing the light beam, reflected and deflected by the deflector, on an effective scanning area of a scanning surface, thereby scanning a plurality of lines on the effective scanning area, wherein the light source remains lit during a period of time between the scanning of each of the plurality of lines on the effective scanning area and wherein a light shield member is positioned between the deflector and the scanning surface to block at least a portion of the light beam emitted during the period of time between the scanning of each of the plurality of lines on the effective scanning area.
A scanning efficiency of the optical scanning device is preferably 70% or higher.
Preferably, the light beam from the first optical system is incident at an oblique angle on a deflection surface of the deflector in a sub scanning cross-sectional plane.
The present invention in another aspect relates to an image forming apparatus and includes an optical scanning device including a first optical system, including a light source, for directing a light beam emitted from the light source to a deflector, and a second optical system for focusing the light beam, reflected and deflected by the deflector, on an effective scanning area of a scanning surface, thereby scanning a plurality of lines on the effective scanning area, wherein the light source remains lit during a period of time between the scanning of each of the plurality of lines on the effective scanning area and wherein a light shield member is positioned between the deflector and the scanning surface to block at least a portion of the light beam emitted during the period of time between the scanning of each of the plurality of lines on the effective scanning area. The image forming apparatus further includes a photoconductive body arranged on the scanning surface of the optical scanning device, a development unit for developing, into a toner image, an electrostatic latent image that has been formed with the light beam scanning the photoconductive body, a transfer unit for transferring the developed toner image onto a paper sheet, and a fixing unit for fixing the transferred toner image onto the paper sheet.
The present invention in yet another aspect relates to an image forming apparatus and includes an optical scanning device including a first optical system, including a light source, for directing a light beam emitted from the light source to a deflector, and a second optical system for focusing the light beam, reflected and deflected by the deflector, on an effective scanning area of a scanning surface, thereby scanning a plurality of lines on the effective scanning area, wherein the light source remains lit during a period of time between the scanning of each of the plurality of lines on the effective scanning area and wherein a light shield member is positioned between the deflector and the scanning surface to block at least a portion of the light beam emitted during the period of time between the scanning of each of the plurality of lines on the effective scanning area. The image forming apparatus further includes a printer controller for converting code data input from an external device into an image signal and feeding the image signal to the optical scanning device.
Preferably, the image is formed through a BAE (Background Area Exposure) process.
The present invention in still another aspect relates to an optical scanning device and incudes a first optical system, including a light source, for directing a light beam emitted from the light source to a deflection surface of a deflector in a beam width wider than the width of the deflection surface in a main scan direction, and a second optical system for focusing the light beam, reflected and deflected by the deflector, on an effective scanning area of a scanning surface, thereby scanning a plurality of lines on the effective scanning area, wherein the light source remains lit during a period of time between the scanning of each of the plurality of lines on the effective scanning area and wherein a light shield member is positioned between the deflector and the scanning surface to block at least a portion of the light beam emitted during the period of time between the scanning of each of the plurality of lines on the effective scanning area.
Preferably, a scanning efficiency of the optical scanning device is 80% or higher.
Preferably, the light beam from the first optical system is incident at an oblique angle on a deflection surface of the deflector in a sub scanning cross-sectional plane.
The present invention in still another aspect relates to an image forming apparatus and includes an optical scanning device including a first optical system, including a light source, for directing a light beam emitted from the light source to a deflection surface of a deflector in a beam width wider than the width of the deflection surface in a main scan direction, and a second optical system for focusing the light beam, reflected and deflected by the deflector, on an effective scanning area of a scanning surface, thereby scanning a plurality of lines on the effective scanning area, wherein the light source remains lit during a period of time between the scanning of each of the plurality of lines on the effective scanning area and wherein a light shield member is positioned between the deflector and the scanning surface to block at least a portion of the light beam emitted during the period of time between the scanning of each of the plurality of lines on the effective scanning area, and wherein the width of a border area between adjacent deflection surfaces of the deflector in a main scan direction is 1% or less of the width of each deflection surface in the main scan direction.
The present invention in still another aspect relates to an image forming apparatus and includes an optical scanning device including a first optical system, including a light source, for directing a light beam emitted from the light source to a deflection surface of a deflector in a beam width wider than the width of the deflection surface in a main scan direction, and a second optical system for focusing the light beam, reflected and deflected by the deflector, on an effective scanning area of a scanning surface, thereby scanning a plurality of lines on the effective scanning area, wherein the light source remains lit during a period of time between the scanning of each of the plurality of lines on the effective scanning area and wherein a light shield member is positioned between the deflector and the scanning surface to block at least a portion of the light beam emitted during the period of time between the scanning of each of the plurality of lines on the effective scanning area, and wherein in a border area between adjacent deflection surfaces of the deflector, one deflection surface extends over the other deflection surface, and the length of the extension in the main scan direction is 5% or less of the beam width of the light beam reflected and deflected from the deflection surface in the main scan direction.
The present invention in still another aspect relates to an image forming apparatus and includes an optical scanning device including a first optical system, including a light source, for directing a light beam emitted from the light source to a deflection surface of a deflector in a beam width wider than the width of the deflection surface in a main scan direction, and a second optical system for focusing the light beam, reflected and deflected by the deflector, on an effective scanning area of a scanning surface, thereby scanning a plurality of lines on the effective scanning area, wherein the light source remains lit during a period of time between the scanning of each of the plurality of lines on the effective scanning area and wherein a light shield member is positioned between the deflector and the scanning surface to block at least a portion of the light beam emitted during the period of time between the scanning of each of the plurality of lines on the effective scanning area. The image forming apparatus further includes a photoconductive body arranged on the scanning surface of the optical scanning device, a development unit for developing, into a toner image, an electrostatic latent image that has been formed with the light beam scanning the photoconductive body, a transfer unit for transferring the developed toner image onto a paper sheet, and a fixing unit for fixing the transferred toner image onto the paper sheet.
The present invention in still another aspect relates to an image forming apparatus and includes an optical scanning device including a first optical system, including a light source, for directing a light beam emitted from the light source to a deflection surface of a deflector in a beam width wider than the width of the deflection surface in a main scan direction, and a second optical system for focusing the light beam, reflected and deflected by the deflector, on an effective scanning area of a scanning surface, thereby scanning a plurality of lines on the effective scanning area, wherein the light source remains lit during a period of time between the scanning of each of the plurality of lines on the effective scanning area and wherein a light shield member is positioned between the deflector and the scanning surface to block at least a portion of the light beam emitted during the period of time between the scanning of each of the plurality of lines on the effective scanning area. The image forming apparatus further includes a printer controller for converting code data input from an external device into an image signal and feeding the image signal to the optical scanning device.
Preferably, the image is formed through a BAE process.
Further objects, features, and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.